if you are looking for top speed... sell you ski, and buy one with a larger engine.
I know it's not what you asked... but you can throw a butt load of $$$$ at it, and it won't be as fast as a stock RXP-X. Then... you will loose all of that with you try to sell it.
Solas impeller, and a tuned pipe from "Factory Pipe." (with the water injection) Add a few parts to make sure the pump is sealed up properly... and that will get you about 6~8 mph up top, and set you back $1000.